One of the first observations that we can make about the human auditory system is that we have two ears. Signals sent from the seperate ears to the brain are combined in two different ways. There is a summing mode, in which information from the two ears in combined so that sounds are louder, and fine distinctions of pitch and information content can be made better, with two ears than with one. There is also a differential mode, in which the information from the two ears is compared. It is this mode which is primarily responsible for our ability to locate sounds in space. This grant application proposes psychological experimental studies of the differential mode. Such studies are timely because in recent years physiologist have worked their way up from the periphery to more central parts of the mammalian auditory system where binaural differences are processed. There is an opportunity to unify the information from these physiological studies with the psychology of two-eared, or binaural, perception. An important goal of these studies is to understand how human listeners can locate sounds in a reflective environment, where they are exposed to a barrage of echos coming from all directions. One step involved here is first to gain a better understanding of the inputs to the ears, by measuring eardrum vibrations with reflected laser light. A second goal is to understand the acoustical difference between sounds which seem to the listener to come from outside and sounds which seem to be located within the head, as often happens with headphone listening. A powerful psychological technique for studying details of the binaural system is obtained from binaural pitch effects. Binaural pitches are those which cannot be heard with one ear alone. They are created in the brain only by combining inputs from both ears.